After you have disabled all the unneeded services on your system, what remains is a core set of connections and programs that you want to keep. However, you are not finished yet: You need to clamp down your wireless network, lock your server physically, and put scanning procedures in place (such as Tripwire and promiscuous mode network monitors).
Securing a Wireless Network
Because wireless networking has some unique security issues, those issues deserve a separate discussion here.
Wireless networking, although convenient, can be very insecure by its very nature because transmitted data (even encrypted data) can be received by remote devices. Those devices could be in the same room; in the house, apartment, or building next door; or even several blocks away. Extra care must be used to protect the actual frequency used by your network. Great progress has been made in the past couple of years, but the possibility of a security breech is increased when the attacker is in the area and knows the frequency on which to listen. It should also be noted that the encryption method used by more wireless NICs is weaker than other forms of encryption (such as SSH) and should not be considered as part of your security plan.
Always use OpenSSH-related tools, such as ssh or sftp, to conduct business on your wireless LAN. Passwords are not transmitted as plain text, and your sessions are encrypted.
The better the physical security is around your network, the more secure it will be (this applies to wired networks as well). Keep wireless transmitters (routers, switches, and so on) as close to the center of your building as possible. Note or monitor the range of transmitted signals to determine whether your network is open to mobile network sniffingnow a geek sport known as war driving. (Linux software is available at http://sourceforge.net/project/showfiles.php?group_id=57253.) An occasional walk around your building not only gives you a break from work, but can also give you a chance to notice any people or equipment that should not be in the area.
Keep in mind that it takes only a single rogue wireless access point hooked up to a legitimate network hub to open access to your entire system. These access points can be smaller than a pack of cigarettes, so the only way to spot them is to scan for them with another wireless device.
Passwords and Physical Security
The next step toward better security is to use secure passwords on your network and ensure that users use them as well. For somewhat more physical security, you can force the use of a password with the LILO or GRUB bootloaders, remove bootable devices such as floppy and CD-ROM drives, or configure a network-booting server for Ubuntu. This approach is not well supported or documented at the time of this writing, but you can read about one way to do this in Brieuc Jeunhomme's Network Boot and Exotic Root HOWTO, available at http://www.tldp.org/HOWTO/Network-boot-HOWTO/
Also, keep in mind that some studies show that as much as 90% of network break-ins are by current or former employees. If a person no longer requires access to your network, lock out access or, even better, remove the account immediately. A good security policy also dictates that any data associated with the account first be backed up and retained for a set period of time to ensure against loss of important data. If you are able, remove the terminated employee from the system before he leaves the building.
Finally, be aware of physical security. If a potential attacker can get physical access to your system, getting full access becomes trivial. Keep all servers in a locked room, and ensure that only authorized personnel are given access to clients.
Configuring and Using Tripwire
Tripwire is a security tool that checks the integrity of normal system binaries and reports any changes to syslog or by email. Tripwire is a good tool for ensuring that your binaries have not been replaced by Trojan horse programs. Trojan horses are malicious programs inadvertently installed because of identical filenames to distributed (expected) programs, and they can wreak havoc on a breached system.
Ubuntu does not include the free version of Tripwire, but it can be used to monitor your system. To set up Tripwire for the first time, go to http://www.tripwire.org, and then download and install an open-source version of the software. After installation, run the twinstall.sh script (found under /etc/tripwire) as root like so:
$ sudo /etc/tripwire/twinstall.sh
The Tripwire site and local passphrases are used to
sign a variety of files, such as the configuration,
policy, and database files.
Passphrases should be at least 8 characters in length
and contain both letters and numbers.
See the Tripwire manual for more information.
Creating key files...
(When selecting a passphrase, keep in mind that good passphrases typically
have upper and lower case letters, digits and punctuation marks, and are
at least 8 characters in length.)
Enter the site keyfile passphrase:
You then need to enter a password of at least eight characters (perhaps best is a string of random madness, such as 5fXkc4ln) at least twice. The script generates keys for your site (host) and then asks you to enter a password (twice) for local use. You are then asked to enter the new site password. After following the prompts, the (rather extensive) default configuration and policy files (tw.cfg and tw.pol) are encrypted. You should then back up and delete the original plain-text files installed by Ubuntu.
To then initialize Tripwire, use its --init option like so:
$ sudo tripwire --init
Please enter your local passphrase:
Parsing policy file: /etc/tripwire/tw.pol
Generating the database...
*** Processing Unix File System ***
Wrote database file: /var/lib/tripwire/shuttle2.twd
The database was successfully generated.
Note that not all the output is shown here. After Tripwire has created its database (which is a snapshot of your file system), it uses this baseline along with the encrypted configuration and policy settings under the /etc/tripwire directory to monitor the status of your system. You should then start Tripwire in its integrity checking mode, using a desired option. (See the TRipwire manual page for details.) For example, you can have Tripwire check your system and then generate a report at the command line, like so:
# tripwire -m c
No output is shown here, but a report is displayed in this example. The output could be redirected to a file, but a report is saved as /var/lib/tripwire/report/hostname-YYYYMMDD-HHMMSS.twr (in other words, using your host's name, the year, the month, the day, the hour, the minute, and the seconds). This report can be read using the twprint utility, like so:
# twprint --print-report -r \
/var/lib/tripwire/report/shuttle2-20020919-181049.twr | less
Other options, such as emailing the report, are supported by Tripwire, which should be run as a scheduled task by your system's scheduling table, /etc/crontab, on off-hours. (It can be resource intensive on less powerful computers.) The Tripwire software package also includes a twadmin utility you can use to fine-tune or change settings or policies or to perform other administrative duties.
Do not ever advertise that you have set a NIC to promiscuous mode. Promiscuous mode (which can be set on an interface by using ifconfig's promisc option) is good for monitoring traffic across the network and can often allow you to monitor the actions of someone who might have broken into your network. The tcpdump command also sets a designated interface to promiscuous mode while the program runs; unfortunately, the ifconfig command does not report this fact while tcpdump is running!
Do not forget to use the right tool for the right job. Although a network bridge can be used to connect your network to the Internet, it would not be a good option. Bridges have almost become obsolete because they forward any packet that comes their way, which is not good when a bridge is connected to the Internet. A router enables you to filter which packets are relayed.